The objective of this project will be to characterize lactate transport by isolated renal tubules of Garter snake (Thamnophis) and of rabbit. Portions of proximal tubule will be dissected from kidney and perfused with C14-lactate and H3-insulin while suspended in a Ringer's bath. Perfusate and bath C14-lactate S.A. and (lactate) will be equal. After perfusion, the tubule will be removed and assayed for cell (lactate). Net lactate transport, fluid reabsorption (Jv) and concentration gradients for lactate will be measured. For Thamnophis, preliminary results suggest that in different tubules lactate may undergo net secretion or reabsorption. More evidence will be gathered to determine if secretion or reabsorption occurs and if either process can be inhibited by changes in medium (Na ion), (K ion), pH, or by probenecid or octanoate. Inhibition of secretion or reabsorption will allow study of the uninhibited transport mechanism in isolation, so that definitive evidence for existence of bidirectional transport may be found, as well as ionic and pH requirements of each transport mechanism. Unidirectional flux studies with H3-lactate in bath (no H3-inulin present) and C14-lactate in perfusate will be done to see if any affects discovered are acting on secretion or reabsorption. Also, effects on Jv of any agents (Na ion, K ion, pH, probenecid or octanoate) changing lactate transport will be examined, and any requirement of lactate for Jv sought. An estimate of the rate of lactate oxidation by isolated tubules will be made to determine the extent to which changes in (lactate) are due to metabolism rather than transport. With rabbit nephron, it is expected that net lactate reabsorption will be found; this process will be quantified with the same methods. An estimate of the lactate metabolism rate in isolated rabbit tubules will be made, and the gradient against which reabsorption occurs determined to see if transport is active or passive. The Tm for reabsorption (if any) will be determined, and affects of changing (lactate) on transport measured. The dependency of lactate transport on (Na ion), (K ion) and pH, and of Jv on (lactate) will also be determined.